The first six U.N. military observers who will monitor a shaky cessation of hostilities in Syria arrived in Damascus late Sunday, a U.N. spokesman said.

“They’ve arrived and they will start work tomorrow morning,” Kieran Dwyer, a spokesman for the U.N. peacekeeping department, told AFP.

The six are the first of 30 monitors who were approved by the UN Security Council on Saturday.

“The other monitors in the advance party are still expected in Syria in coming days,” Dwyer added.

The first group will set up a headquarters and prepare routines so that the mission can verify that a cessation of hostilities, started Thursday, between President Bashar al-Assad’s forces and opposition fighters is holding.

Syrian forces reportedly killed five civilians in shelling of rebel areas and clashes with gunmen in the hours before the arrival of the first monitors. U.N. leader Ban Ki-moon raised strong concerns about the continuing violence.

U.N.-Arab League envoy Kofi Annan eventually wants more than 200 observers in Syria, but the Security Council has said the full mission can only go if the violence halts.

Syria warned on Sunday that its forces will respond to “intensified” rebel attacks, even as U.N. military observers were headed for the unrest-hit country to monitor a shaky ceasefire.

“Armed terrorist groups have intensified their attacks in a hysterical way against the army, law enforcement forces, and civilians, in conjunction with the approval of the U.N. Security Council resolution to send monitors to Syria,” a military official said, quoted by state television.

“(Security forces), based on their duty to protect civilians and the country, will stop terrorist groups from continuing their criminals acts and the killing of civilians,” the state news agency SANA said.

Meanwhile, Syrian government forces shelled the city of Homs on Sunday, resident opposition activists and a rights activist said, as a six-person advance party of U.N. observers was due to arrive in Syria to monitor a ceasefire meant to start four days ago.

“The bombardment of Khaldiyeh intensified this morning with an average of three shells a minute,” the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.

The rebel district of Bayada was also shelled, Abdel Rahman said, adding that it was the fiercest shelling of Homs since a U.N.-backed ceasefire went into force at dawn on Thursday.

U.N. chief Ban Ki-moon voiced concern on Sunday over the Syrian regime’s shelling of Homs and urged the government to do everything to maintain the ceasefire.

“I am very much concerned about what has happened since yesterday and today,” Ban said after talks with Belgian Prime Minister Elio Di Rupo in Brussels.

The security forces now control some 70 percent of the flashpoint city, which has seen some of the biggest losses of life of the 13-month uprising in Syria.

An initial team of monitors is due to arrive in Syria on Sunday evening and will be deployed on Monday, the spokesman for international mediator Kofi Annan said.

They will be joined by at least two dozen more in coming days in line with a U.N. Security Council resolution adopted on Saturday authorizing their deployment, Ahmad Fawzi said.

“The first batch of six U.N. observers arrives tonight, they will be on the ground in blue helmets tomorrow (Monday),” Fawzi told Reuters in Geneva. “They will be quickly augmented by up to 25-30 from the region and elsewhere.”

Meanwhile, Ban said that he would make proposals by next Wednesday regarding the full observer mission, expected to number about 250. Their deployment requires a second Security Council resolution.

British Foreign Secretary William Hague said that the number of unarmed observers was insufficient and had to be “beefed up.”

“This number of people cannot possibly effectively monitor what is happening in the whole country,” he said in an interview with Sky News, adding that 30 monitors could, however, quickly visit areas where they are reports of ceasefire breaches.

“The plan will be for a much larger (team), more in the hundreds, of monitors to follow them provided the (ceasefire) plan is being implemented by all concerned,” he said.

Three civilians died in shelling of Homs on Saturday, among 14 people killed nationwide ahead of a U.N. Security Council vote approving the dispatch of the observer mission to monitor the shaky truce.

With a cease-fire barely holding and the deployment of unarmed foreign observers expected to ease but not end months of violence, world powers are still struggling to find a longer-term strategy for Syria.

In theory, the world’s most powerful countries, the Syrian government and much of the Syrian opposition have all signed up to a multipoint plan formulated by former U.N. chief Kofi Annan. But the reality remains much more complex.

While Moscow and Beijing have repeatedly said they want to avoid a Libya-style externally backed regime change, the United States, Britain and France still say they want Assad gone.

“The main focus at the moment is the... rapid deployment of monitors,,” said one Western official on condition of anonymity. “That’s a priority, but it’s not the only one... at the end of the day, do not see a future for Syria with Assad in charge. We are in this for the long haul.”

But officials concede they have few immediate tools with which to make that happen. Even a U.S. plan to provide “non-lethal” support to opposition fighters could end up being shelved, some suspect, largely because the rebels remain so disunited and ineffective.

U.S. pressure, insiders say, has already deterred Saudi Arabia and Qatar from making good on long-running talk they might provide weapons.

For now, Annan, Western powers and their Arab allies say Assad remains in breach of much of the peace plan. Privately, many Western diplomats worry his strategy may be to give just enough to drive a wedge between them and a much more reluctant Russia and China.

“The game for the weekend is watching whether the ceasefire holds and the monitors are approved,” said another Western official. “After that it gets more complicated.”

The most realistic immediate hope, officials say, would be that Assad's forces cease use of heavy weaponry.

But if an earlier Arab league monitoring attempt in Syria -- or other previous similar missions in Sri Lanka and Kosovo -- are anything to go by, unarmed observers might struggle to stop killings, abductions and use of snipers.

Certainly, few believe Syria will genuinely follow through to withdraw troops from urban areas or allow peaceful protest. For world powers, the true challenge will come if the monitors report a fall in violence but accuse Syria of ignoring other areas of Annan’s plan.